The present invention relates to a method and an arrangement for burst mode control to achieve effective radio transmission of data between a fixed station and at least one mobile station at one of a plurality of carrier frequencies, this data is transmitted in active time slots using a time-division multiplex method (TDMA), these slots each being followed by an inactive time slot.
The DECT Digital Enhanced Cordless Telecommunication Standard was adopted at the start of the 1990""s in order to replace the various existing analogue and digital Standards in Europe. This was the first common European Standard for cordless telecommunications. A DECT network is a microcellular, digital mobile radio network for high subscriber densities. It is primarily designed for use in buildings. However, it is also possible to use the DECT Standard outdoors. The capacity of the DECT network of around 10,000 subscribers per square kilometer provides, from the cordless standard, ideal access technology for network operators. According to the DECT Standard, it is possible to transmit both voice and data signals. Thus, cordless data networks can also be built on a DECT base.
The DECT Standard will be explained in more detail in the following text with reference to FIG. 2. A digital, cordless telecommunications system for ranges of less than 300 m has been standardized for Europe under the designation DECT. In conjunction with the switching function of a telecommunications installation, this system is suitable for mobile telephone and data traffic in an office building or on a commercial site. The DECT functions supplement a telecommunications installation, and thus make it the fixed station FS of the cordless telecommunications system. Digital radio links between the fixed station FS and a maximum of 120 mobile stations MS can be produced, monitored and controlled on up to 120 channels.
A maximum of ten different carrier frequencies (carriers) are used for transmission in the frequency range from 1.88 GHz to 1.9 GHz. This frequency-division multiplex method is called FDMA (Frequency Division Multiple Access).
Twelve channels are transmitted successively in time on each of the ten carrier frequencies using the time-division multiplex method TDMA (Time Division Multiple Access). Cordless telecommunication in accordance with the DECT Standard using ten carrier frequencies with in each case twelve channels per carrier frequency provides a total of 120 channels. Since one channel is required, for example, for each voice link, there are 120 links to the maximum of 120 mobile stations Ms. The duplex method (TDD) is used for on the carriers. Once the base stations has transmitted twelve channels (channels 1-12), it switches to received, and receives twelve channels in the opposite direction (channels 13-24).
A time-division multiplex frame thus comprises 24 channels (see FIG. 2). In this case, channel 1 to channel 12 are transmitted from the fixed station FS to the mobile stations MS, while channel 13 to channel 24 are transmitted in the opposite direction, from the mobile stations MS to the fixed station FS. The frame duration is 10 ms. The duration of a channel (time slot) is 417 xcexcs. 320 bits of information (for example voice) and 104 bits of control data (synchronization, signalling and error check) as well as 56 bits of so-called guard (protection) field are transmitted in this time. The useful bit rate for a subscriber (channel) results from the 320 bits of information within 10 ms. It is thus 32 kilobits per second.
Integrated modules have been developed to carry out the DECT functions for fixed and mobile stations. In this case, the fixed station and the mobile station carry out similar functions. One of these integrated modules is the RF module, i.e., the module which carries out the actual function of receiving and transmitting the RF band.
It is known for so-called fast hopping RF modules to be used, (RF modules which can carry out a carrier frequency change very quickly, for example from one tome slot or channel to the next). These fast hopping RF modules are intrinsically very complex and costly. Thus, in practice, so-called slow hopping RF modules are mainly used, (RF modules which require a certain amount of time to program the carrier frequency for the next time slot). In practice, the time period which the slow hopping RF module requires to program the carrier frequency corresponds essentially to the time period of a time slot in the DECT Standard. This means that, after each active time slot, (in which data are transmitted), a so-called inactive time slot (blind slot) in which no data can be transmitted, must follow. This means that, in practice, only six links are available on one carrier frequency to the DECT Standard, instead of the twelve possible links.
A DECT channel is defined by its time slot and its carrier frequency. The organization to reuse physical channels is carried out by means of dynamic channel selection. Meaning that there is no need for any complex frequency planning, as in cellular systems. To set up a link, the signal levels of all the channels are measured continuously, and the interference-free channels are controlled in a channel list (channel map). While a link exists, the signal levels of all the channels and the reception quality continue to be monitored. If this monitoring indicates that the channel currently being used has been transmitted at a carrier frequency which was subject to interference (for example, as a result of the influence of a transmission at the same carrier frequency from or to another fixed station), another carrier frequency is automatically selected for the next active time slot, and is entered in the channel list as being interference-free. Alternatively, the carrier frequency change can also be carried out after each frame.
As a further option, a carrier frequency change may always take place after a predetermined time period, such as a time slot or a frame, (designated xe2x80x9cfrequency hopping spread spectrumxe2x80x9d.)
In other countries, the transmission conditions and standards may differ. For example, in the USA, the normal DECT band between 1.88 and 1.90 GHz cannot be used for transmission, but the generally accessible 2.4 GHz ISM band (Industrial, Scientific, Medical) is available instead. Furthermore, changes would have to be carried out for matching to the national Standards, such as the American Standard xe2x80x9cFCC part 15xe2x80x9d (Federal Communications Commission). This American Standard describes the transmission method, transmission powers and available bandwidth allowed for the radio interface.
In the DECT Standard, in addition to the 320 information bits mentioned above, each time slot also contains another 104 bits required for signal transmission, as well as 56 bits in the guard field, so that each time slot contains a total of 480 bits. This results in a data rate of (24xc3x97480 bits)/10 ms=)1,152,000 bits/s. A data rate at this level is pointless in the American ISM band, since the bandwidth required per usable channel would be too large.
The problem thus exists of using components which have been developed for the DECT Standard in other transmission conditions as well, for cost reasons, while at the same time making it possible to use the available bandwidth efficiently.
EP-0 767 551 discloses a method for increasing the load and, thus, the capacity of the DECT system, whereby the ISM frequency band in the 2.4 GHz range is used for information transmission in addition to the DECT frequency band between 1.880 and 1.900 GHz, and the FHSS method (Frequency Hopping Spread Spectrum) or the DSSS method (Direct Sequence Spread Spectrum) is also applied.
GB-2 295 930 discloses a TDMA radio system based on the frequency hopping method wherein a frequency change is implemented in the guard period between two time slots. Radio devices of the TDMA radio system respectively comprise two RF modules (synthesizers) for this frequency change in the guard period between two time slots. While the time slot-related transmission of information occurs via one RF module, the respective radio device is set to the next time slot frequency with the other RF module.
The present invention thus has the object of providing a mobile radio and a method for digital radio transmission of data which allow effective use of the bandwidth of a TDMA system in a simple manner. The method and the arrangement should particularly allow, cost-effective use of slow hopping RF modules.
A main idea of the present invention is in this case to refine the burst mode controller in a mobile radio.
According to the present invention, a mobile radio is provided for radio transmission in time-division multiplex frames. The time-division multiplex frames in each case alternately have active time slots; in which data are transmitted, and inactive time slots; in which no data are transmitted. The time duration of these active time slots is twice the duration of the inactive time slots. The mobile radio according to the present invention has a burst mode controller which presets the structure of the time-division multiplex frames for transmission, as well as a clock preset device, which presets the clock rate for the burst mode controller. The clock rate which the clock preset device presets for this burst mode controller is twice as high during the inactive time slots as during the active time slots.
The clock rate during the inactive time slots can be chosen to be equal to the clock rate in the known DECT Standard. The clock rate during the active time slots is half the DECT clock rate.
The mobile radio can be set to receive/transmit in a 2.4 GHz band.
According to the present invention, a method is also provided for radio transmission between a fixed station and at least one mobile station using time-division multiplex frames, the time-division multiplex frames in each case alternately having active time slots, in which data are transmitted, and inactive time slots, in which no data are transmitted. The time duration of these active time slots is twice the duration of the inactive time slots. A burst mode controller presets the time-division multiplex frames for transmission, and a clock device in turn presets the clock rate for the burst mode controller. According to the invention, the clock device presets a clock rate for the burst mode controller during the inactive time slots which is twice as high as the clock rate during the active time slots.
The clock rate during the inactive time slots may be chosen to be equal to the clock rate in the known DECT Standard.
The transmission can take place in a 2.4 GHz band.
A time frame for transmission may contain four active time slots for transmission from the fixed station to a mobile station, followed by four time slots for transmission from the mobile station to the fixed station.